The present invention relates to a guidance method and system suitable for use with autonomously guided and man-in-the-loop guided vehicles where the presence of obstacles and/or threats must be considered in guiding the vehicle to a destination.
During the travel of an autonomously guided or man-in-the-loop guided vehicle towards a destination, there may be obstacles, impediments and/or threats present in the path of travel which must be avoided which otherwise would prevent the vehicle from reaching its destination or would cause damage to the vehicle. Various guidance augmentation concepts have been suggested for avoiding obstacles, operational boundaries, and/or threats. Generally, these concepts have been developed with the approach such that if no impediments are present, then the travel of the guided vehicle is not changed. However, if impediments are present, these concepts provide the guidance system with directions diverting the vehicle around the impediment to allow the vehicle to proceed to the desired destination.
These avoidance methods have been used, for instance, in the field of tactical missiles. There, during the launch, flight, and intercept phases of flight, there are numerous constraints on the guidance system that result from obstacles or operational limitations that must be accommodated in addition to the requirement that the missile be guided so as to hit the target. Some of these constraints include maneuverability constraints based on dynamic, kinematic, and/or mechanical constraints of the vehicle; operational envelope constraints such as maximum altitude or minimum altitude limitations on the vehicle; obstacle constraints, such as friendly airborne assets; and, threat constraints such as enemy assets which may target the vehicle in an attempt to destroy the vehicle. All of these factors are significant considerations in shaping the trajectory of the missile during its flight. Various textbook guidance methods that have been proposed will be briefly discussed below. These methods, however, do not take into account all of those constraints and therefore would benefit from augmentation in some form prior to implementation in a real world application.
A first known method for accommodating guidance constraints is known as a waypoint method. This guidance method uses waypoints or intermediate points along a trajectory. One such concept is described in the article xe2x80x9cObstacle-Avoidance Automatic Guidance: A Concept Development Studyxe2x80x9d by Victor H. L. Cheng, published in AIAA Journal, Paper Number 88-4189-CP, 1988. The basic idea behind the waypoint concept is that if an obstacle, operational boundary, or threat is perceived, a set of waypoints is constructed such that an avoidance path can be achieved. This concept is illustrated in FIG. 1. As shown in FIG. 1, a vehicle 10 can be navigated from a starting point 110 to a destination 140 via waypoints 120 and 130. To generate an efficient trajectory, the waypoints must be selected carefully. The waypoint method is suitable for applications where the impediments are stationary and are not dynamic.
Another known avoidance method is to use optimal guidance strategies to steer a vehicle around obstacles or boundaries. One such approach is described in the article xe2x80x9cControl Theoretic Approach to Air Traffic Conflict Resolutionxe2x80x9d, AIAA Journal, Paper Number 93-3832-CP, 1993. This method involves the definition of a cost function that measures the quality or goodness of a particular trajectory and which optimizes the cost function over a set of possible trajectories. The construction of the cost function is generally based on the dynamics or relative motion of the obstacles, operational boundaries, and/or threats to the missile. Generally, this method must be solved numerically and can require large amounts of real time processing to solve. Also, the addition of multiple obstacles, operational boundaries, and/or dynamic threats in the operation or flight significantly increases the complexity of the cost function and thus further increases processing requirements.
Another avoidance method relies upon mathematical representations of potential fields. In this approach, sources, which are potential field elements that provide a mathematically repelling force, can be used individually or as a surface to provide a range dependent force in order to push the missile guidance away from the obstacle. Sources have a unique quality in that the amount of repelling force is inversely proportional to the distance from the source. For example, a missile that is close to a source will be pushed away with greater force than a missile that is far from the source. Because of this range dependent characteristic, and because sources are computationally efficient to use, sources can be used in a large number of avoidance applications. However, source methods have a drawback that, if they are not modified, they will affect the guidance commands by directing the vehicle away from the obstacle throughout the trajectory of the vehicle because the source has an infinite range of influence.
A modification to the source approach adds a range boundary beyond which the source will not affect the vehicle. One such approach is described in xe2x80x9cGeneration of Conflict Resolution Maneuvers for Air Traffic Managementxe2x80x9d, by Claire, et al., IEEE Journal, Paper No. 0/7803-4119-8, 1997. In this approach, the method will push the guidance command away from the obstacle only when the missile is inside the range boundary and will not affect the vehicle when it is outside the boundary. One drawback to this method is that the guidance command is mathematically discontinuous across the boundary. Discontinuous commands are undesirable because they can cause instabilities in the guidance solution in the circumstance when multiple obstacles, boundaries, or threats are encountered. Another drawback to this method is in regard to threat avoidance. Specifically, this method will tend to direct a vehicle along the direction of the velocity of the threat in order to avoid the threat. However, if the threat has sufficient velocity to overtake the vehicle and the source potential is located on the vehicle, the threat can nevertheless have a successful intercept. The Claire, et al. article also mentions the use of vortex potential field elements in the construction of an avoidance solution. Vortex elements are essentially planar rotation elements that push the field in a rotational direction about a point. Vortex elements are not easily adapted to multi-dimensional spaces above two dimensions, because they are two-dimensional elements. To apply a vortex element in a three-dimensional space would require knowledge of the direction which the vortex would act about. In a dynamic situation, the addition of a vortex element would limit the number of evasive solutions and would hinder the performance of an avoidance algorithm. Nevertheless, two-dimensional applications of vortex elements can be appropriate in circumstances such as traffic management applications where the flow of vehicles conforms to pre-established guidelines.
Another known avoidance method employs specific geometric boundary shapes to describe how an avoidance maneuver is to be performed. These geometric methods use the surface tangents from the geometric boundary to provide a direction of avoidance for the avoidance maneuver. One such avoidance approach is described in xe2x80x9cA Self-Organizational Approach for Resolving Air Traffic Conflictsxe2x80x9d, pages 239-254, by Martin S. Eby, published in The Lincoln Laboratory Journal, Volume 7, No. 2, 1994. Such an approach is illustrated in FIG. 2, where a vehicle 10 is traveling in direction of arrow D and must avoid obstacle plane 200. A geometric boundary shape 210 is defined around obstacle plane 200 at a radius rD which is the desired miss distance. In the case where the path of the subject plane 10 to its destination 220 intersects the geometric boundary shape 210, an avoidance maneuver is necessary. In this circumstance, an avoidance velocity vector solution VS is found, in part, by forming a tangent 230 to the geometric boundary 210. A significant limitation to this method with regard to tactical missiles is that if the missile gets inside of the avoidance geometric boundary, then the tangent vector to the surface no longer exists. The Claire, et al. article also discusses how the size of the geometric boundary shape can be reduced in order to allow for calculation of a tangent. Specifically, in the case where the missile gets inside of the avoidance geometric boundary, the geometric boundary shape is reduced until the missile is returned to the exterior of the geometric boundary. The reduction of the size of the geometric boundary can work for obstacle avoidance because as a subject vehicle approaches the surface geometry, the guidance direction becomes more aggressive in an avoidance direction.
One shortcoming of the method occurs when trying to provide an operational boundary such as a maximum altitude for a vehicle. One representation of an absolute altitude ceiling is a single sphere which surrounds the earth at a specified altitude. Such a sphere, however, would immediately cause a problem with the above-described method because the vehicle would always be inside the geometric boundary and thus no tangent could be formed. Such a method would also have difficulty with an altitude ceiling limitation if a maximum altitude were represented by a plane. In this case, the geometric tangent method would provide an avoidance command that was always parallel to the plane. As a result, the desired avoidance command would be ignored at all times.
The tangent geometric boundary avoidance technique is also inadequate in the circumstance of launching a missile from an aircraft. Specifically, the tangent method cannot accommodate the situation where the missile is inside the geometric shape which surrounds the vehicle to be avoided. Thus, the geometric shape must be made very small. However, if the geometric shape surrounding a launching aircraft is made small enough to allow the tangent method to work, the geometric shape would be smaller than the aircraft to be avoided, no avoidance commands would be issued, and the missile would be able to fly into the launching aircraft.
Accordingly, in view of the above, what is needed is an improved avoidance system and method which can address the shortcomings outlined above.
An object of the present invention is to provide an avoidance system and method which addresses both static and dynamic obstacles and which does not require specialized algorithms for dynamic obstacles.
Another object of the present invention is to provide an obstacle avoidance system and method which provide a deterministic avoidance command and which does not require an iterative minimization algorithm for a solution.
A further object of the present invention is an avoidance system and method which comprehends the situation where the vehicle is inside of a geometric boundary shape.
According to the present invention, the foregoing objects are attained by blending together a distance-based field element (such as source, sink, or vortex), a geometric boundary surface normal and tangent, an original guidance command, and the distance to the obstacle to produce an avoidance command that will direct the missile away from the obstacle and proceed in the general direction of the target destination.
The avoidance commands are constructed in a fashion such that, in the case where a vehicle is very close to an obstacle, the avoidance command directs the vehicle in a direction to prevent collision. When a vehicle approaches a geometric boundary, the avoidance command is blended from an anti-intercept direction to a direction that is a function of a surface tangent, such that the avoidance command is continuous across the geometric boundary. Outside the geometric boundary, the guidance command is blended with a vector that is a function of the surface tangent taken in a distance dependent fashion to provide an avoidance direction along the desired direction of travel. As a result, the inclusion of obstacles and/or threats in the operational space of a vehicle will produce an alteration to the nominal guidance commands yet still allow the vehicle to travel to its intended final destination or final state. In the absence of obstacles, operational boundaries or threats, the present invention will not alter the original guidance commands and the vehicle will travel according to the original guidance implementation.
This invention may be applied in applications such as autonomous vehicles as well as vehicles employing man-in-the-loop guidance. Specifically, such as in the situation of a flight director, an operator of the vehicle may be informed of a proper guidance adjustment required to avoid an obstacle.